1. Field of the Invention
The present invention relates to a novel aluminum nitride sintered body. More particularly, the invention relates to an aluminum nitride sintered body which has excellent light transmission properties and can be preferably used particularly as a translucent cover for a light source of high luminous efficiency.
2. Background Art
As light transmitting materials, materials that are transparent to visible light, such as transparent resin, glass, quartz and light transmitting alumina, have been heretofore properly used according to the use environment, cost, etc. For example, as a cover (window material) for a light source of low energy intensity or as a light transmitting material by which light of wavelength rather including no ultraviolet light is transmitted, a transparent resin or a glass is used. As a translucent cover for a light source rather including ultraviolet light or a light source having high energy intensity and thereby having a high temperature when it is used, a material using quartz or alumina is used. As a translucent cover for a light source using a corrosive gas such as a halogen gas, an alumina material having corrosion resistance is used.
Recently, improvement of light sources has been further promoted, and light sources of higher luminous efficiency have been produced. For example, light sources using, as luminescent materials, enclosures containing halides (particularly iodides and bromides) of metals, such as Na, Sc, Sn, Th, Tl, In, Li Tm, Ho and Dy, are known. Enhancement of luminance of light sources, however, increases heat generated, so that in the light sources of high luminous efficiency, materials of light transmitting members such as covers applied to the light sources become problems. That is to say, although the alumina material has resistance to halogen gas, the resistance is still insufficient. Moreover, because of low thermal conductivity of 30 W/mK, heat dissipation of a light source becomes insufficient, and therefore, there is a fear that the life of the light source is shortened. Further, there is another problem of poor color rendering properties because the temperature of an arc tube surface becomes ununiform. Accordingly, a light transmitting member having resistance to halogen gas and high thermal conduction properties is desired.
In order to solve the above problems, aluminum nitride that is excellent in heat resistance, thermal conduction and mechanical strength properties has been proposed as a material of a light transmitting window material that transmits infrared rays or laser beam (see patent document 1). In this publication, it is disclosed that when a powdery raw material having a specific particle diameter of the material powder, a specific content of metallic impurities and a specific content of oxygen is sintered in an inert atmosphere of 1700 to 2100° C., an AlN sintered body showing a transmittance of 75% in the wavelength region of 0.2 μm to 30 μm is obtained.
Further, an arc tube having a translucent cover (hollow tube) composed of an aluminum nitride sintered body which is produced by the use of a raw material aluminum nitride powder having such a particle size distribution that the amount of particles having diameters of 0.3 D to 1.8 D (D: average particle diameter) is not less than 70% is disclosed (see patent document 2). In a working example of this publication, an aluminum nitride sintered body having a total light transmittance of 84% is shown.
According to the techniques described above, it is possible to produce an AlN sintered body improved in the light transmission properties. However, there is yet room for improvement in the light transmittance. That is to say, in the case where an aluminum nitride sintered body is used as a translucent cover, its transmittance in the visible region of 400 nm to 800 nm is desired to exceed 85% taking reflectance into consideration, but the transmittance (400 nm to 800 nm) of an aluminum nitride sintered body obtained by the above-mentioned publicly known techniques is at most 85%. In contrast therewith, the alumina material has a transmittance exceeding 95% though it is inferior to the aluminum nitride sintered body in the resistance to halogen gas and the thermal conductivity. In comparison with the alumina material, therefore, the aluminum nitride sintered body is requested to be further improved in the transmittance.
Moreover, with regard to the light transmission properties in the ultraviolet region, there is yet room for improvement in the build-up properties of light transmittance of the conventional light transmitting aluminum nitride sintered body. When the aluminum nitride sintered body is used as a translucent cover, the build-up properties of light transmittance are important properties to realize high light transmittance in the wide wavelength region of the ultraviolet region to the infrared region.
In a patent document 3, there is disclosed an aluminum nitride sintered body characterized in that the oxygen concentration is held down to not more than 400 ppm, the metallic impurity concentration is held down to not more than 150 ppm, the carbon concentration is held down to not more than 200 ppm, and the sintered body has an average crystal grain diameter of 2 μm to 20 μm. In this sintered body, the inclination of the spectral curve in the wavelength region of 260 to 300 nm is not less than 1.0 (%/nm), the light transmittance in the wavelength region of 400 to 800 nm is not less than 86%, and the wavelength at which 60% of a light transmittance is reached in the spectrum is 400 nm.
Patent document 1: Japanese Patent Laid-Open Publication No. 26871/1990
Patent document 2: Japanese Patent Laid-Open Publication No. 193254/1985
Patent document 3: Japanese Patent Laid-Open Publication No. 119953/2005